The SALICIDE (self aligned silicide) process is widely used in the manufacture of integrated circuits. It allows conductive contact to be made to very small areas of silicon (and/or areas of silicon that are very close together) separated by areas of silicon oxide or nitride.
The principle on which the process operates is that several metals, of which titanium is a prime example, react very rapidly when heated in contact with a silicon surface and are converted to their silicide. If the metal is in contact with silicon oxide or nitride during the same heat treatment, no reaction occurs. After etching in a suitable selective etch, all unreacted metal can be removed, leaving behind only the silicide which, as noted above, is limited to the silicon areas. The silicide turns out to be a reasonably good electrical conductor, particularly if it is given a second heat treatment.
Although the SALICIDE process as described above has been widely applied, it does have certain associated problems. In particular, during the first heat treatment, while the metal is in contact with the silicon oxide, a small amount of silicon gets leached out of the oxide or nitride so that a thin layer of silicide may, in fact, form on its surface. This remains in place after the selective etch (to remove unreacted metal) and forms a bridge between the intended conductive areas.
Several solutions to this problem are currently in use within the semiconductor industry. One of these involves the use of nitrogen to suppress the outdiffusion of silicon from the silicon oxide. One way of implementing this is to ion implant nitrogen into the silicon oxide prior to the deposition of the silicide forming metal. While this is effective for stopping bridging, it can lead to degradation of the silicon areas through raising their sheet resistance. Another approach is to do the silicide formation in a nitrogen ambient. This approach involves an extra process step and is also difficult to control since the nitrogen needs to diffuse through the full thickness of the metal before it can reach the silicon oxide surface and, en route, may convert much of the metal to the nitride (which has high resistivity).
Thus, what is needed is a way to deliver a controlled quantity of nitrogen to the silicon oxide surface, without transforming the metal layer to its nitride and without introducing an additional process step into the overall manufacturing process. The present invention teaches how this may be done using titanium as the silicide forming metal, but it will be understood that the process of the present invention could be modified to similarly improve the SALICIDE process when used with other silicide forming metals such as cobalt or tungsten.
In the course of searching the prior art, several references that teach ways to deposit titanium and titanium nitride films were found but none of these teach (or could be combined to teach) the process of the present invention. For example, Wang et al. (U.S. Pat. No. 5,508,212 April 1996) coat a silicon surface with titanium and then convert the latter to titanium nitride through ion implantation. After an RTA, excess unreacted TiN is removed in the usual way.
Wang (U.S. Pat. No. 5,604,155 February 1997) deposits a titanium layer followed by an aluminum layer. After heating, Ti--Al alloy is formed which allows the removal of precipitates and eliminates bridging.
Chen (U.S. Pat. No. 5,462,895 October 1995) uses CVD (chemical vapor deposition) to form an adhesive layer inside a contact hole that is later filled with tungsten. By changing the ratio of ammonia to titanium chloride in the precursor gases, a two layer film of stoichiometric titanium nitride over titanium-rich titanium nitride is formed. In U.S. Pat. No. 5,525,543 (June 1996), which is a divisional of U.S. Pat. No. 5,462,895, Chen describes several additional embodiments.
Akahori (U.S. Pat. No. 5,508,066 April 1996) teach the advantages of depositing titanium by means of plasma enhanced CVD.